With the advancements in Digital Sampling and Semiconductor technology, Arbitrary Waveform Generators (AWGs) with high sampling rate Digital to Analog Converters (50 GS/s and 25 GS/s) have become available on the market. These high bandwidth AWGs can create wide bandwidth IQ (baseband) signals required for coherent optical signal communication receiver testing, where I represents the in-phase signal and Q represents the quadrature signal.
AWGs that generate test signals for wide band applications such as optical communication are ideally required to have a flat magnitude and linear phase frequency response in the band of interest. In addition to flatness, skew and gain between the I and Q channels must be matched when testing wideband IQ signals. The quality of a digitally modulated signal is measured using Error Vector Magnitude (EVM). Any distortion in the signals due to skew or gain mismatches between the I and Q channels will increase the signal's EVM, indicating that the digitally modulated signal has poor quality.
The test signal must undergo de-embedding or pre-compensating for any magnitude distortions, phase distortions, or skew and gain imbalance introduced by the test setup before being used to qualify a Device Under Test (DUT). Typically, pre-distortion coefficients are determined before testing a DUT, and may be used to de-embed or pre-compensate the test signal during testing.
One method used to obtain the pre-distortion coefficients is a two-step process, wherein the frequency response of each of I and Q channels are measured independently. The frequency response is then inverted and two real-valued pre-distortion coefficients are obtained. These pre-distortion coefficients can be used by correction filters which are applied on the I and Q signals separately.
Another method obtains complex pre-distortion coefficients by directly measuring the complex frequency response of the I and Q signals.
The main disadvantage of the above methods lies in that they obtain frequency responses without considering the skew and magnitude imbalance between the channels. The skew and magnitude have a significant role in degradation of EVM of the signals. For example, measuring the complex frequency response without correcting for skew imbalances between the channels may result in incorrect phase measurements.
To obtain the skew and gain imbalance, another prior art method may be used. For example, a known signal such as a step signal or a sinusoidal signal may be generated simultaneously from both the I and Q channels. The skew and gain imbalances between the channels are measured manually and corrected by either mathematically shifting and scaling one of the signals, or changing the clock phase between the AWGs and the amplitude control of the channels.
One drawback of this prior art method results from the user having to manually obtain the skew and gain correction values. This often becomes a time consuming method of trial-and-error, and prone to user measurement errors.
Automated methods exist to synchronize and adjust the skew between AWG channels, but they do not account for distortions caused by cables running to the Device Under Test (DUT). Currently, no methods are known for automatically adjusting gain imbalances between the channels.